docs: add information about ipc sysctls limitations

After 25b21cb2f6d6 ("[PATCH] IPC namespace core") and 4e9823111bdc
("[PATCH] IPC namespace - shm") the shared memory page count stopped being
global and started counting per ipc namespace.  The documentation and
shmget(2) still says that shmall is a global option.

shmget(2):

SHMALL System-wide limit on the total amount of shared memory, measured in
units of the system page size.  On Linux, this limit can be read and
modified via /proc/sys/kernel/shmall.

I think the changes made in 2006 should be documented.

Link: https://lkml.kernel.org/r/09e99911071766958af488beb4e8a728a4f12135.1705333426.git.legion@kernel.org
Signed-off-by: Alexey Gladkov <[email protected]>
Signed-off-by: Eric W. Biederman <[email protected]>
Acked-by: "Eric W. Biederman" <[email protected]>
Link: https://lkml.kernel.org/r/ede20ddf7be48b93e8084c3be2e920841ee1a641.1663756794.git.legion@kernel.org
Cc: Christian Brauner <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Joel Granados <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Manfred Spraul <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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